This invention relates generally to precipitators for removing particulate contaminants from a gaseous stream, and more particularly to an inertial-electrostatic precipitator in which ionized particles are caused to migrate toward a downwardly-flowing liquid film formed on the inner surface of a collector tube, the migration resulting from the combined action of electrostatic and centrifugal force whereby the stream may be purified in the course of its passage through a relatively short tube.
Electrostatic precipitators function to separate contaminating particles or droplets of a semi-solid or solid nature from a gaseous stream. Such precipitators are especially helpful in removing finer particles (less than 40.mu.). In one known form of electrostatic precipitator of the dry type, the gases to be purified are conveyed through a collector tube where they are subjected to an electrostatic field which ionizes the particles and causes migration toward the inner surface of the collector tube, thereby separating the particles from the gas flowing through the tube. With continued operation of a dry precipitator, the particles accumulate on the wall of the collector tube and it becomes necessary, therefore, at fairly frequent intervals, to shut down the precipitator in order to permit removal of the agglomerated particles.
With a wet wall precipitator of the type disclosed, for example, in the deSeversky U.S. Pat. No. 3,716,966, a uniform film of downwardly flowing water is formed on the inner wall of the collector tube, the film serving to continuously flush away the contaminants, thereby obviating the need to interrupt the precipitator operation.
While large particles suspended in an air stream are also ionized in an electrostatic field, their migration velocity is usually so low that they tend to be swept out by the gas stream before reaching the electrostatic collecting surfaces. Hence conventional electrostatic precipitators are less effective with respect to coarse particles carried by the contaminated gas.
The use of centrifugal separators or cyclonic collectors for separating dust particles and other particulate contaminants of 25.mu. or larger from a gaseous stream is well known. In order, therefore, to effectively remove both large and small particles from a gaseous stream, one may feed the gaseous stream first through a cyclonic collector or inertial dust separator stage to extract the large particles from the stream and then through an electrostatic precipitator stage to extract the small particles therefrom. Thus U.S. Pat. No. 3,315,445 to deSeversky discloses a pollution control system in which gas scrubber and wet electrostatic precipitator stages are intercoupled in cascade relation so as to remove the full spectrum of contaminants from the stream.
The drawback to a gas scrubber, a centrifugal collector or other form of coarse particle separator operating in series with an electrostatic precipitator is that this combination requires elaborate and costly duct work to convey the gaseous stream through the serially connected stages. Moreover, the gas emitted from the first stage may not have a velocity profile appropriate to the second stage. To overcome this drawback, in the deSeversky U.S. Pat. No. 3,315,445, the scrubber stage is designed to modify the gas flow pattern so that gas emerging from the gas scrubber has a laminar flow characteristic and a uniform velocity profile. Nevertheless, the resultant combined structure is relatively tall and massive, for the electrostatic precipitator stage is stacked directly above the scrubber stage.
While a tall, vertically-mounted precipitator arrangement represents no serious problem when this structure is installed outside a building whose contaminated exhaust must be purified before being discharged into the atmosphere, in some cases the need exists for interior precipitator installation, such as in cotton processing plants and in other dust-producing facilities as well as in so-called "clean rooms" for processing electronic components where it is necessary to recirculate the atmosphere through a filter or other dust in order to remove particulate contaminants therefrom. In such cases, the available headroom may be insufficient to accommodate a vertically-mounted precipitator installation.